Although I see you’ve already made your decision, I can answer this question as I have both. For its size, the Spark mini sounds pretty good, but it doesn’t hold a candle to the sound quality of the 40. I’m going to have to research the Nux, if you chose it over all the features of the Spark. I hadn’t heard of it before today so you’ve peaked my curiosity.

What makes the Spark perfect for me is the ability to download any guitar sound I want. There’s a huge community that create custom sounds, most based on the specific sound of a specific artist and song. So when I want to sound like AC/DC Back in Black I just search in the app for cloud tones of AC/DC (or Back in Black), preview a few (which can be done without actually downloading them but you can still play your guitar to hear the tone) then pick my favorite to add to the amp.

Edit: The Nux looks like the better deal as you get the features of a Spark, but with a free wireless guitar jack. I had to buy a wireless system to be able to use my Spark without a guitar cable anchoring me down and will never go back to using cables. So it looks like you made the right choice (or at least the one I would have if I had never heard either IRL).